Numerous types of vehicles are used for traveling over a variety of surfaces. For example, all terrain vehicles and snowmobiles may be used to traverse smooth roads, small to medium bumps, very large bumps, and frozen or unfrozen terrain all in a single excursion. To obtain the optimum ride quality for each set of conditions, a different ratio of travel between the vehicle and the biasing mechanism on the suspension system is required. The ratio of shock absorber and/or spring displacement divided by the total vertical displacement of the suspension system relative to the vehicle chassis at a selected location (hereinafter referred to as motion ratio) for optimum ride control varies greatly depending on the terrain conditions and speed at which the vehicle is traveling.
The importance of a proper motion ratio can be illustrated by the snowmobile 10 of FIG. 1. The snowmobile 10 has a body frame or chassis 12 that mounts a seat 14 on an upper side thereof Seated on the snowmobile seat 14, the driver manually steers the vehicle 10 by a handlebar assembly 18 that is secured to a steering shaft 20 that extends through a compartment 22 for the internal combustion engine 24. The steering shaft 20 is operatively connected to a pair of steerable skis 28 through a steerable linkage preferably arranged so that the inside cornering ski 28 turns at a greater angle than the outside ski 28 so as to provide comfortable steering.
An endless track 16 driven by a main drive wheel 40 operatively connected to the internal combustion engine 24 rotates around a suspension system 26. The suspension system 26 includes a slide rail 30 connected to the chassis 12 by a front suspension arm 32 and a rear suspension arm 34. At least one biasing/dampening mechanism 36 is provided for biasing the slide rail 30 away from the chassis 12. In the embodiment illustrated in FIG. 1, lower end 38 of the rear biasing/dampening mechanism 36, is pivotally connected to the slide rail 30 through a mechanical adjustment mechanism (not shown) to provide for adjusting the location of the lower end 38 over a distance, and thus, adjusting the motion ratio. In one embodiment, adjustment of the lower end 38 is made within a slot 42.
There is no easy way to adjust the motion ratio for a specific terrain without stopping the vehicle and manually making the adjustment. For example, in the context of the snowmobile of FIG. 1, the driver may move from a smooth, groomed trail to a very rough terrain. If the suspension has been delivering optimum ride quality on the smooth trail, the motion ratio may need to be increased to provide optimum ride quality on the rough terrain. For example, the vehicle will periodically bottom out at lower speeds than would otherwise be able to be maintained with a proper motion ratio. The driver is faced with a choice of either stopping the snowmobile and crawling under the chassis to make the adjustment or enduring the consequences of a less than optimum motion ratio.
A smooth and controlled ride over varied terrain is one of the most important handling qualities of snowmobiles, as well as a variety of other recreational vehicles. The suspending forces of the vehicle suspension system with regard to any bumps is directly affected by the speed of the shock absorber as well as the displacement of the biasing/dampening mechanism. Both of these factors are controlled by the motion ratio. The higher the motion ratio, the greater the resistance of the suspension system to vertical displacement during compression travel. The lower the motion ratio, the less resistance provided by the suspension system to vertical displacement on compression travel. All other parameters remaining the same, a motion ratio that is high enough to withstand bottoming of the chassis against the suspension system in rough terrain with large bumps will also deliver a rough, less comfortable ride on smoother terrain. In the opposite conditions, a motion ratio low enough to deliver a comfortable ride on smooth terrain will periodically bottom out on rough terrain.
U.S. Pat. No. 3,115,945 illustrates a chassis support apparatus having a pivotally mounted cushion cylinders that may be adjusted more and less vertical to adjust for heavy or light loads. The cushion cylinders are pivotally attached to a moveable pivot block that is engaged with a threaded transverse member rigidly mounted to the frame. Consequently, the adjustment mechanism is located on the frame.